This invention relates to devices for measuring the dimension of airborne fibers and more particularly, to non-contact optical devices for measuring the diameter and length of flowing fibers.
Existing airborne fiber dimension measuring devices typically require that the fibers in the sampled air be separated, aligned, and analyzed individually. Furthermore, some devices require multiple sensors which determine airborne fiber dimensions by analyzing the residual amount of direct light from collimated light beams that have been attenuated or otherwise disturbed by a passing fiber. The accuracy of devices requiring multiple sensors can be adversely affected by alignment errors, calibration drift, component degradation and the like.
Two examples of existing methods and apparatus for the measurement of entities in fiber samples include, for example, U.S. Pat. No. 5,430,301 to Shofner, et al. (1995), entitled xe2x80x9cApparatus and Methods for Measurement and Classification of Generalized Neplike Entities in Fiber Samplesxe2x80x9d (Shofner I); and U.S. Pat. No. 5,270,787 to Shofner, et al. (1993) entitled, xe2x80x9cElectro-Optical Methods and Apparatus for High Speed, Multivariate Measurement of Individual Entities in Fiber or Other Samplesxe2x80x9d (Shofner II). See also MIE Fiber Monitor Model FM-7400 User""s Manual by MIE, Inc., Billerica, Mass.
In Shofner I, multiple sensors are provided for measuring fiber characteristics in a sample of textile material, including small clumps or entanglements of fiber known as neps. Although this apparatus and method may be suitable for determining the characteristics of neps from textile samples, it is generally unsuitable for characterizing airborne fibers, such as glass fibers, having a diameter of less than about 10 microns.
Similarly, the device in Shofner II employs multiple sensors to directly measure the amount of light remaining from a collimated light beam which has been at least partially extinguished by the passage of a fiber between the source of the collimated light beam and the sensor. Shofner II analyzes ribbon-shaped cotton fibers with a typical width of approximately 20 microns. In addition, Shofner II analyzes the diffraction pattern that results from a cotton fiber passing through the sensing zone. This apparatus and device is also unsuitable for certain other fibers, particularly those of narrow diameter, such as glass fibers having a diameter of less than about 5-10 microns due to its non-monotonic response to fiber diameter.
What is therefore needed is an airborne fiber dimension measuring device that can accurately characterize the dimensions of small-diameter fibers.
This invention provides devices and methods for determining the dimension of airborne fibers. The device includes flow means for providing a laminar flow to at least a portion of a group of fibers in a air sample. These aligned fibers are then illuminated with a light source to create scattered light. A light detector is then used for sensing a portion of the scattered light and for generating an output from which a dimension of a first of these fibers can be provided.
This invention takes advantage of the characteristics of scattered light to produce very accurate measurements of fiber dimensions, such as diameter and length. In preferred embodiments, scattered light is collected from a slotted opening at an angle of about 60xc2x0 to about 120xc2x0 relative to the direction of the light source to produce an approximate monotonic voltage amplitude range, indicative of a fiber diameter.
This invention also provides a method for measuring a dimension of an airborne fiber. The method includes providing a fiber-containing air sample having a laminar flow. This air sample is thereafter contacted by a light beam to produce scattered light. The scattered light is sensed and an electrical output is produced which is representative of a sensed portion of the scattered light. This electrical output is then processed to produce a perceptible indication of a dimension of at least a first fiber in said sample.